Intervertebral prosthetic disc with metallic core

ABSTRACT

A prosthetic disc for insertion between adjacent vertebrae includes a core having upper and lower curved surfaces and upper and lower plates. At least one of the curved surfaces of the core is metallic, and in some embodiments the entire core is metallic. Each plate has an outer surface which engages a vertebra and a metallic inner curved surface which is shaped to slide over one of the curved surfaces of the core. In some embodiments, the center of rotation of the core is free to move relative to the upper and lower metallic plates. In some embodiments, one or more channels extend across one or both of the curved surfaces of the core for allowing passage of bodily fluid to promote lubrication between the core and at least one of the plates.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. Nos.10/855,253 and 10/855,817, both of which were filed on May 26, 2004, andboth of which are hereby incorporated fully by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to medical devices and methods. Morespecifically, the invention relates to intervertebral disc prostheses.

Back pain takes an enormous toll on the health and productivity ofpeople around the world. According to the American Academy of OrthopedicSurgeons, approximately 80 percent of Americans will experience backpain at some time in their life. In just the year 2000, approximately 26million visits were made to physicians' offices due to back problems inthe United States. On any one day, it is estimated that 5% of theworking population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunctionof one or more intervertebral discs. Intervertebral discs are the softtissue structures located between each of the thirty-three vertebralbones that make up the vertebral (spinal) column. Essentially, the discsallow the vertebrae to move relative to one another. The vertebralcolumn and discs are vital anatomical structures, in that they form acentral axis that supports the head and torso, allow for movement of theback, and protect the spinal cord, which passes through the vertebrae inproximity to the discs.

Discs often become damaged due to wear and tear or acute injury. Forexample, discs may bulge (herniate), tear, rupture, degenerate or thelike. A bulging disc may press against the spinal cord or a nerveexiting the spinal cord, causing “radicular” pain (pain in one or moreextremities caused by impingement of a nerve root). Degeneration orother damage to a disc may cause a loss of “disc height,” meaning thatthe natural space between two vertebrae decreases. Decreased disc heightmay cause a disc to bulge, facet loads to increase, two vertebrae to rubtogether in an unnatural way and/or increased pressure on certain partsof the vertebrae and/or nerve roots, thus causing pain. In general,chronic and acute damage to intervertebral discs is a common source ofback related pain and loss of mobility.

When one or more damaged intervertebral discs cause a patient pain anddiscomfort, surgery is often required. Traditionally, surgicalprocedures for treating intervertebral discs have involved discectomy(partial or total removal of a disc), with or without fusion of the twovertebrae adjacent to the disc. Fusion of the two vertebrae is achievedby inserting bone graft material between the two vertebrae such that thetwo vertebrae and the graft material grow together. Oftentimes, pins,rods, screws, cages and/or the like are inserted between the vertebraeto act as support structures to hold the vertebrae and graft material inplace while they permanently fuse together. Although fusion often treatsthe back pain, it reduces the patient's ability to move, because theback cannot bend or twist at the fused area. In addition, fusionincreases stresses at adjacent levels of the spine, potentiallyaccelerating degeneration of these discs.

In an attempt to treat disc related pain without fusion, an alternativeapproach has been developed, in which a movable, implantable, artificialintervertebral disc (or “disc prosthesis”) is inserted between twovertebrae. A number of different intervertebral disc prostheses arecurrently being developed. For example, the inventors of the presentinvention have developed disc prostheses described in U.S. patentapplication Ser. Nos. 10/855,817 and 10/855,253, previously incorporatedby reference. Other examples of intervertebral disc prostheses are theLINK® SB Charité disc (provided by DePuy Spine, Inc.) Mobidisk®(provided by LDR Medical (www.ldrmedical.fr)), the Bryan Cervical Disc(provided by Medtronic Sofamor Danek, Inc.), the ProDisc® or ProDisc-C®(from Synthes Stratec, Inc.), and the PCM disc (provided by Cervitech,Inc.). Although existing disc prostheses provide advantages overtraditional treatment methods, improvements are ongoing.

One type of intervertebral disc prosthesis generally includes upper andlower prosthesis plates or shells, which locate against and engage theadjacent vertebral bodies, and a low friction core between the plates.In some designs, the core has upper and lower convexly curved surfaces,and the plates have corresponding, concavely curved recesses whichcooperate with the curved surfaces of the core. This allows the platesto slide over the core to allow required spinal movements to take place.Some type of movement limiting structure is provided, to prevent thecore from slipping out between the plates. Typically, the plates aremade of one or more metals, and the core is made of a polymericsubstance.

One of the challenges in designing an intervertebral disc prosthesis isto prevent or reduce wear and tear of the core. In many prostheticdiscs, the plates are metallic, and the core is made of a polymericmaterial. Although a core made of a polymeric or other resilientmaterial may last many years, it would be advantageous to have coresthat could last even longer, so that patients (especially youngerpatients) would not be faced with the possibility of repeat surgeries toreplace disc prostheses. At the same time, longer-lasting cores shouldstill allow for a desirable range and ease of motion of the twovertebrae about the intervertebral disc prosthesis.

Therefore, a need exists for improved intervertebral disc prostheses.Ideally, such improved prostheses would provide improved resistance towear and tear while also allowing a desired amount of vertebral movementabout the prosthesis. At least some of these objectives will be met bythe present invention.

2. Description of the Background Art

A number of exemplary intervertebral disc prostheses are listed above.Published US patent applications 2002/0035400A1 and 2002/0128715A1describe disc implants which comprise opposing plates with a corebetween them over which the plates can slide. The core receives one ormore central posts, which are carried by the plates and which locate inopposite ends of a central opening in the core. Such arrangements limitthe load bearing area available between the plates and core.

Other patents related to intervertebral disc prostheses include U.S.Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437;5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296;5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130;6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797;6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068.Other patent applications related to intervertebral disc prosthesesinclude U.S. Patent Application Publication Nos.: 2003/0009224;2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982;2001/0016773 and 2003/0100951. Other related patents include WO01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508,EP 1344493, EP 1417940, EP 1142544, and EP 0333990.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention, a prosthetic disc for insertionbetween adjacent vertebrae includes a core having upper and lower curvedsurfaces and upper and lower plates. At least one of the curved surfacesof the core is composed of or covered by a metal, which forms a metallicportion of the core. Each plate has an outer surface which engages avertebra and a metallic inner curved surface which is shaped to slideover one of the curved surfaces of the core. The center of rotation ofthe core is free to move relative to the upper and lower metallicplates. Thus, the plates can slide freely in all directions, not beinglimited to movement in a single direction as with the prior art. Metalendplates combined with a core having at least one metallic surface willhelp prevent wear and tear of the disc prosthesis.

“Curved surfaces” of the core and/or the plates typically means thatsuch surfaces are spherical. However, it is also contemplated thatother, non-spherical surface shapes may be used, such as but not limitedto ovoid, crowned, domed or other complementary surface shapes whichprovide for the desired freedom of movement of the plates relative tothe core. A center of rotation of the core that is free to move relativeto the upper and lower plates means that the core is not fixed in animmobile state to either of the plates. Thus, the core may movelaterally or “float” relative to the plates, and the plates may movefreely over the core.

The upper and lower plates may be made of any suitable metal, metalalloy or combination of metals or alloys. In some embodiments, forexample, the plates may be made of cobalt chrome molybdenum, titanium,stainless steel or some combination thereof. In some embodiments,titanium plates are used, and these plates may optionally include innersurfaces of titanium nitride and outer surfaces that are aluminum oxideblasted to create micro-concavities. In another embodiment, cobaltchrome plates are used, with the outer surfaces being blasted withaluminum oxide and then coated with a titanium plasma spray. In someembodiments, the plates comprise an MRI-compatible material, such astitanium, coupled with a hardened material, such as cobalt chromemolybdenum. Such materials may be coupled using any suitable means, suchas welding, laminating, slip fitting, interference fitting, adhesion,welding, molding, heating and cooling one material to attach it toanother, or the like. Some plates include a coating or material on theinner surfaces for reducing friction and/or wear and tear, such as atitanium nitride surface.

The metallic portion of the core may also be made of any suitable metal,alloy or combination of metals or alloys. In various embodiments, forexample, one curved surface of the core, both curved surfaces of thecore, or the entire core may comprise cobalt chrome molybdenum,titanium, stainless steel or some combination thereof. In someembodiments, the core and the upper and lower plates may be made of thesame metal, while in alternative embodiments they may be made ofdifferent metals. In some embodiments, the core may comprise acombination of metallic and non-metallic substances, such as metal andceramic, polymer, a combination of polymers, or the like. In suchembodiments, the curved surfaces of the core may be laminated or coatedin metal, or metallic curved surface portions may be attached to thenon-metallic portions. In one embodiment, the core is hollow, withmetallic curved surfaces.

The core may have any suitable configuration or shape. In oneembodiment, the core comprises two oppositely facing, convex,low-friction, metallic or metal-covered surfaces which slidably engagethe inner curved surfaces of the upper and lower plates. One or both ofthe upper and lower curved surfaces of the core may optionally includeat least one channel on the surface(s) for allowing passage or intrusionof bodily fluid to promote lubrication between the core and at least oneof the plates. In some embodiments, two or more channels are included onat least one of the core surfaces. Such channels, for example, may beoriented perpendicularly across the upper and/or lower surfaces tointersect. In some embodiments, each of the upper and lower surfaces ofthe core includes at least one channel. The core may have additionalsurface features in various embodiments, such as but not limited tothreads for screwing into complementary threads on the upper and/orlower plates.

In some embodiments, the present invention further provides restrainingstructure on one or both of the plates or the core to hold the coreagainst the curved surface of at least one of the plates during slidingmovement of the plates over the core. For example, one or moreperipheral restraining structures may be included. The peripheralrestraining structure defines a limit or boundary for movement of thecore relative to at least one of the upper and lower plates. Within sucha peripheral boundary, however, movement of the core relative to theplate will preferably be unconstrained. That is, movement of the corerelative to the plate may occur in any direction without significantinhibition or friction. The core will preferably not be attached toeither the upper or lower plate, and the plates will thus be able tofreely articulate relative to each other over the core, which provides alow friction bearing surface for each plate.

An advantage of the structure thus described is that the surface contactarea between the core and each of the upper and lower plates may bemaximized. By providing only a peripheral restraint, as opposed forexample to grooves and keys on the surface of the core and plates, thewidth or diameter of the core relative to the size of the plate may bemaximized. Moreover, the surfaces of the core and the plates whichcontact each other may be made smooth and free from other structure(s)that might adversely affect performance. In the preferred embodiments,both the curved surfaces of the plates and the corresponding surfaces ofthe core will be spherical sections. The use of spherical surfacespromotes free, unconstrained relative motion of the plates and the corein all directions.

In some embodiments, the peripheral restraining structure limitsrelative inclination of the plates during sliding movement of the platesover the core, usually by defining a stop structure. In otherembodiments, the peripheral restraining structure lifts one side of thecore relative to an opposite side of the core during sliding movement ofthe plates over the core. The peripheral restraining structure itselfmay take any of a number of different forms. In one embodiment, forexample, the restraining structure comprises a ring structure on atleast one of the upper and lower plates and an annular structure on atleast a portion of the periphery of the core. In one embodiment, thering structure is adapted to engage and restrain the annular structureon the core. For example, the ring structure may comprise a flange whichdefines an overhang over at least a portion of the periphery of one ofthe plates. The overhang of the flange will receive the annularstructure on the core to provide an interference fit which retains thecore against the curved surface of the plate but allows the core toslide freely and in an unconstrained manner within the limit or boundarydefined by the flange. The annular structure on the core may be a rimwhich extends continuously or discontinuously (preferably continuously)around a lateral circumference of the core. By providing a rim which hasa width, usually a diameter, which is slightly greater than thecorresponding width of an inner edge of the flange at one point, thecore will be held in place and will not be dislodged from the cavitydefined by the ring structure in normal use.

In an alternative embodiment, the annular structure on the core may havea width or outer diameter that is slightly smaller than an innerdiameter of the ring structure (such as a flange) on the upper or lowerplate. Thus, the annular structure on the core may be passed through thering structure to engage the core with the upper or lower plate. Thecore is then held in place, relative to the upper or lower plate, viaforces applied by the adjacent vertebrae and surrounding soft tissuestructures. Essentially, this embodiment is analogous to aball-and-socket joint. Such an embodiment may be advantageous for easeof assembly of prosthetic disc with a metallic core and metallicendplates.

Usually, the flange or other ring structure as well as the rim or otherannular structure will be formed continuously about the periphery of theplate and core, respectively. Alternatively, however, either or both ofthe annular structure and the ring structure could be formeddiscontinuously. That is, so long as at least some portion of the ringstructure and the annular structure remain engaged during all expectedgeometries and uses of the prosthetic disc, the objective of holding thecore against the curved surface of the plate will be met.

Optionally, in some embodiments the outer surfaces of the upper andlower plates have at least one surface feature for promoting attachmentof the outer surfaces to the vertebrae. For example, such surfacefeatures may include a plurality of serrations disposed along the outersurfaces. Some embodiments include additional or alternative features onthe outer surfaces for enhancing attachment of the prosthesis tovertebral bone, such as a material or coating, like a titanium plasmaspray. Multiple micro-concavities may be formed on the outer surfaces,for example by aluminum oxide spraying, to further enhance attachment.Additionally or alternatively, the surface features may include at leastone fin disposed on each of the outer surfaces. In some embodiments, thefin includes at least one hole for further promoting attachment to thevertebrae. Fins may extend vertically from their corresponding outersurfaces at right angles, or alternatively the fins may extend fromtheir corresponding outer surface at angles other than 90°. Fins mayalso have any suitable orientation relative to the anterior-posterioraxis of the prosthesis. For example, a fin may extend in a straight linefrom anterior to posterior, without being angled. Alternatively, the finmay be rotated or angled away from the anterior-posterior axis at anysuitable angle between 0° and 180°. In one embodiment, each fin isdisposed in a lateral orientation on the outer surfaces.

In another aspect of the present invention, a prosthetic disc forinsertion between adjacent vertebrae includes a core having upper andlower curved surfaces, and upper and lower plates. Again, at least oneof the curved surfaces of the core is composed of a metal, and in someembodiments the entire core is metallic. Additionally, each of thecurved surfaces includes at least one surface channel for allowingpassage of bodily fluid to promote lubrication between the core and theplates. Each plate has an outer surface which engages a vertebra and ametallic inner curved surface which is shaped to slide over one of thecurved surfaces of the core. In a preferred embodiment, the center ofrotation of the core is free to move relative to the upper and lowermetallic plates. Any of the features described above may also beincorporated in various embodiments.

In another aspect of the present invention, a prosthetic disc forinsertion between adjacent vertebrae includes a metallic core havingupper and lower curved surfaces and upper and lower metallic plates.Each plate has an outer surface which engages a vertebra and an innercurved surface which slides over the curved surface of the core. Thecenter of rotation of the core is free to move relative to the upper andlower metallic plates. In some embodiments, the core and the plates aremade of the same metal. Some embodiments further include one or morechannels on each curved surface of the core, on one or more of theplates, or both, for promoting lubrication.

In another aspect of the present invention, a method for assembling aprosthetic disc for insertion between adjacent vertebrae involvesmovably coupling a core with a first metallic endplate to form aninterference fit between the core and the first endplate and contactingthe core with a second metallic endplate. At least one of the curvedsurfaces of the core is metallic, as described above. In someembodiments, coupling the metallic core with the first metallic endplateinvolves heating the first endplate sufficiently to cause it to expand,inserting a portion of the core into the expanded endplate restrainingstructure, and allowing the first endplate to cool, thus contracting toform the interference fit around the portion of the core. In analternative embodiment, coupling the core with the first endplatecomprises forming the endplate around the core. Alternatively, couplingthe core with the first endplate may involve screwing the core into thefirst endplate via complementary threads on the core and the firstendplate. In some embodiments, coupling the core with the first endplatemay involve engaging a peripheral protrusion of the core with aperipheral restraining structure of the first endplate.

In another aspect of the present invention, a method for implanting anintervertebral disc prosthesis between adjacent vertebrae comprisesimplanting an upper metallic plate against a lower surface of an uppervertebral body, implanting a lower metallic plate against an uppersurface of a lower vertebral body, and disposing a core (at least onecurved surface of which is metallic) between the upper and lower plates.The core floats with a mobile center of rotation between sphericalcavities in each of the upper and lower plates. In some embodiments, theplates restrain peripheral movement of the core using at least oneperipheral restraining member. In some embodiments, disposing the corebetween the plates involves passing an annular structure of the corethrough a ring structure of one of the plates. In some embodiments,implanting each of the plates comprises sliding a fin on each plate intoa corresponding groove formed in its respective vertebral body. The finmay slide into the groove in any suitable direction, such asposterior-anterior, anterior-posterior, lateral, or any angled directionbetween an anterior-posterior orientation and a lateral orientation.Optionally, implanting may further involve contacting textured outersurfaces of the upper and lower plates with the upper and lower surfacesof the vertebral bodies.

These and other aspects and embodiments will be described in furtherdetail below, with reference to the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1A ae cross-sectional anterior views of a prosthetic discwith the prosthesis plates and core in vertical alignment, according toembodiments of the present invention;

FIG. 2 is a side view of the prosthetic disc in FIG. 1 after slidingmovement of the plates over the core;

FIG. 3 is a side view of the prosthetic disc in FIG. 1 aftertranslational movement of the plates relative to the core;

FIG. 4 is a side view of the prosthetic disc in FIG. 1 with theprosthesis plates and core in vertical alignment;

FIG. 5 is a perspective view of a core of a prosthetic disc, accordingto one embodiment of the present invention; and

FIG. 6 is a superior view of an upper plate of a prosthetic disc,according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention generally provide for anintervertebral disc prosthesis having upper and lower plates and a corehaving at least one metallic surface. In various embodiments, the coremay have a mobile center of rotation, one or more surface channels forpromoting passage of lubricating fluid, or both. FIGS. 1-6 generallydemonstrate one embodiment of such a prosthesis. The general principlesof the present invention, however, may be applied to any of a number ofother disc prostheses, such as but not limited to the LINK® SB Charitédisc (provided by DePuy Spine, Inc.) Mobidisk® (provided by LDR Medical(www.ldrmedical.fr)), the Bryan Cervical Disc (provided by MedtronicSofamor Danek, Inc.), the ProDisc® or ProDisc-C® (from Synthes Stratec,Inc.), and the PCM disc (provided by Cervitech, Inc.).

That being said, and with reference now to FIGS. 1-4 a prosthetic disc10 for intervertebral insertion between two adjacent spinal vertebrae(not shown) suitably includes an upper plate 12, a lower plate 14 and acore 16 located between the plates. The upper plate 12 includes an outersurface 18 and an inner surface 24 and may be constructed from anysuitable metal, alloy or combination of metals or alloys, such as butnot limited to cobalt chrome molybdenum, titanium (such as grade 5titanium), stainless steel and/or the like. In one embodiment, typicallyused in the lumbar spine, the upper plate 12 is constructed of cobaltchrome molybdenum, and the outer surface 18 is treated with aluminumoxide blasting followed by a titanium plasma spray. In anotherembodiment, typically used in the cervical spine, the upper plate 12 isconstructed of titanium, the inner surface 24 is coated with titaniumnitride, and the outer surface 18 is treated with aluminum oxideblasting. An alternative cervical spine embodiment includes no coatingon the inner surface 24. In other cervical and lumbar disc embodiments,any other suitable metals or combinations of metals may be used. In someembodiments, it may be useful to couple two materials together to formthe inner surface 24 and the outer surface 18. For example, the upperplate 12 may be made of an MRI-compatible material, such as titanium,but may include a harder material, such as cobalt chrome molybdenum, forthe inner surface 24. Any suitable technique may be used to couplematerials together, such as snap fitting, slip fitting, lamination,interference fitting, use of adhesives, welding and/or the like. Anyother suitable combination of materials and coatings may be employed invarious embodiments of the invention.

In some embodiments, the outer surface 18 is planar. Oftentimes, theouter surface 18 will include one or more surface features and/ormaterials to enhance attachment of the prosthesis 10 to vertebral bone.For example, the outer surface 18 may be machined to have serrations 20or other surface features for promoting adhesion of the upper plate 12to a vertebra. In the embodiment shown (seen best in FIG. 6), theserrations 20 extend in mutually orthogonal directions, but othergeometries would also be useful. Additionally, the outer surface 18 maybe provided with a rough microfinish formed by blasting with aluminumoxide microparticles or the like. In some embodiments, the outer surfacemay also be titanium plasma sprayed to further enhance attachment of theouter surface 18 to vertebral bone.

The outer surface 18 may also carry an upstanding, vertical fin 22extending in an anterior-posterior direction. The fin 22 is pierced bytransverse holes 23. In alternative embodiments, the fin 22 may berotated away from the anterior-posterior axis, such as in alateral-lateral orientation, a posterolateral-anterolateral orientation,or the like. In some embodiments, the fin 22 may extend from the surface18 at an angle other than 90°. Furthermore, multiple fins 22 may beattached to the surface 18 and/or the fin 22 may have any other suitableconfiguration, in various embodiments. In other embodiments, the fin 22In some embodiments, such as discs 10 for cervical insertion, the fins22, 42 may be omitted altogether.

The inner, spherically curved concave surface 24 is formed at a central(from right to left), axial position with a circular recess 26 asillustrated. At the outer edge of the curved surface 24, the upper plate12 carries peripheral restraining structure comprising an integral ringstructure 26 including an inwardly directed rib or flange 28. The flange28 forms part of a U-shaped member 30 joined to the major part of theplate by an annular web 32. The flange 28 has an inwardly tapering shapeand defines upper and lower surfaces 34 and 36 respectively which areinclined slightly relative to the horizontal when the upper plate 12 isat the orientation seen in FIG. 1. An overhang 38 of the U-shaped member30 has a vertical dimension that tapers inwardly as illustrated.

The lower plate 14 is similar to the upper plate 12 except for theabsence of the peripheral restraining structure 26. Thus, the lowerplate 14 has an outer surface 40 which is planar, serrated andmicrofinished like the outer surface 18 of the upper plate 12. The lowerplate 14 optionally carries a fin 42 similar to the fin 22 of the upperplate. The inner surface 44 of the lower plate 14 is concavely,spherically curved with a radius of curvature matching that of the innersurface 24 of the upper plate 12. Once again, this surface may beprovided with a titanium nitride or other finish.

At the outer edge of the inner curved surface 44, the lower plate 14 isprovided with an inclined ledge formation 46. Alternatively, the lowerplate 14 may include peripheral restraining structure analogous to theperipheral restraining structure 26 on the upper plate 12.

The core 16 of the disc 10 is at least partially made of one or moremetals, alloys or a combination of metals or alloys. For example, metalsused to form all or part of the core 16 may include but are not limitedto cobalt chrome molybdenum, titanium (such as grade 5 titanium),stainless steel and/or the like. In some embodiments, the core 16 may bemade of the same material as the upper plate 12 and the lower plate 14,which may help resist oxidation of metallic surfaces of the disc 10. Inalternative embodiments, the core 16 may be made of differentmaterial(s) than the plates 12, 14. In the embodiment shown, the core 16has identical upper and lower spherically curved convex surfaces 48, 50.At least one of the curved surfaces 48, 50 is metallic or covered inmetal. In some embodiments, the entire core 16 is metallic, while inother embodiments the curved surfaces 48, 50 may be coated or laminatedwith metal, or one or more metallic surfaces may be otherwise attachedto the core 16. In some embodiments, the core 16 is made of a polymer orceramic, with attached metallic curved surfaces 48, 50. Alternatively,the core 16 may be a hollow metallic structure. The radius of curvatureof these surfaces matches the radius of curvature of the inner surfaces24, 44 of the upper and lower plates 12, 14. The curved surfaces areaccordingly complementary.

The core 16 is symmetrical about a central, equatorial plane 52 whichbisects it laterally. (Although in other embodiments, the core 16 may beasymmetrical.) Lying on this equatorial plane is an annular recess orgroove 54 which extends about the periphery of the core. The groove 54is defined between upper and lower ribs or lips 56. When the plates 12,14 and core 16 are assembled and in the orientation seen in FIG. 1, theflange 28 lies on the equatorial plane and directly aligned with thegroove 54. The outer diameter 58 of the lips 56 is preferably veryslightly larger than the diameter 60 defined by the inner edge of theflange 28. In some embodiments, the core 16 is movably fitted into theupper plate 12 via an interference fit. To form such an interference fitwith a metal core 16 and metal plate 12, any suitable techniques may beused. For example, the plate 12 may be heated so that it expands, andthe core 16 may be dropped into the plate 12 in the expanded state. Whenthe plate 12 cools and contracts, the interference fit is created. Inanother embodiment, the upper plate 12 may be formed around the core 16.Alternatively, the core 16 and upper plate 12 may include complementarythreads 59, 61 as shown in FIG. 1A, which allow the core 16 to bescrewed into the upper plate 12, where it can then freely move.

In an alternative embodiment (not shown), the outer diameter 58 of thelips 56 may be very slightly smaller than the diameter 60 defined by theinner edge of the flange 28. In such embodiments, the core 16 and theplates 12, 14 are not coupled via an interference fit but are insteadcoupled via forces applied by the vertebral column itself, thus actinganalogously to a ball-and-socket joint.

Referring now to FIG. 5, in some embodiments, the core 16 includes oneor more surface channels 102 for allowing passage of fluid along thecontact surfaces 104 of the core 16. Bodily fluids and/or injected fluidmay pass through such a channel 102, between the core 16 and the upperand lower plates 12, 14, to promote lubrication between the contactsurfaces 104 of the core and their corresponding surfaces on the upperand lower plates 12, 14. Any number, pattern, shape, depth, width orlength of surface channels 102 may be included on a contact surface 104,in various embodiments. In some embodiments, for example, channels 102may have a depth of about 3 mm or less, and more preferably about 2 mmor less, and even more preferably about 1 mm or less. Surface channels102 may have a cross-sectional shape that is curved, rectangular,V-shaped or any other suitable shaped. Furthermore, surface channels 102may be disposed on the contact surface(s) 104 of the core 16 in ahelical pattern (as shown) or in any other suitable pattern, such ascircular, rectangular, curved, one or more straight, parallel lines, twoor more perpendicular lines, or the like. Surface channels 102 helpprevent sticking or loss of freedom of motion (seizing) between the core16 and the plates 12, 14 which may occur otherwise due to metal-on-metalcontact.

In some embodiments, one or both of the inner surfaces 24, 44 of theupper and lower plates 12, 14 may also include one or more surfacechannels 25, 45 as shown in FIG. 1. Again, such channels may have anysuitable configuration, size, number and shape, and may assist inpromoting lubrication between the core 16 and the upper and lower plates12, 14.

The central axis of the disc 10 (the axis passing through the centers ofcurvature of the curved surfaces) is indicated with the referencenumeral 62. As shown in FIG. 1, the disc 10 may be symmetrical about acentral anterior-posterior plane containing the axis 62. Referring toFIG. 4, in some embodiments the axis 62 is posteriorly disposed, i.e. islocated closer to the posterior limit of the disc than the anteriorlimit thereof.

In use, the disc 10 is surgically implanted between adjacent spinalvertebrae in place of a damaged disc. The adjacent vertebrae areforcibly separated from one another to provide the necessary space forinsertion. The disc is inserted, normally in a posterior direction, intoplace between the vertebrae with the fins 22, 42 of the plates 12, 14entering slots cut in the opposing vertebral surfaces to receive them.During and/or after insertion, the vertebrae, facets, adjacent ligamentsand soft tissues are allowed to move together to hold the disc in place.The serrated and microfinished surfaces 18, 40 of the plates 12, 14locate against the opposing vertebrae. The serrations 20 and fins 22, 42provide initial stability and fixation for the disc 10. With passage oftime, enhanced by the titanium surface coating, firm connection betweenthe plates and the vertebrae will be achieved as bone tissue grows overthe serrated surface. Bone tissue growth will also take place about thefins 22, 40 and through the transverse holes 23 therein, furtherenhancing the connection which is achieved.

In the assembled disc 10, the complementary and cooperating sphericalsurfaces of the plates and core allow the plates to slide or articulateover the core through a fairly large range of angles and in alldirections or degrees of freedom, including rotation about the centralaxis 62. FIGS. 1 and 4 show the disc 10 with the plates 12 and 14 andcore 16 aligned vertically with one another on the axis 62. FIG. 2illustrates a situation where maximum anterior flexion of the disc 10has taken place. At this position, the upper rib 56 has entered thehollow 38 of the U-shaped member 30, the lower surface of the rib 56 hasmoved into contact with the upper surface 34 of the flange 28, theflange having moved into the groove 54, and the lower surface 36 of theflange has moved into contact with the upper surface of the ledgeformation 46, as will be seen in the encircled areas 69. Abutmentbetween the various surfaces prevents further anterior flexure. Thedesign also allows for the inner extremity of the flange 28 to abutagainst the base of the groove 54, thereby limiting further relativemovement between the core and plate. A similar configuration is achievedin the event of maximum posterior flexure of the plates 12, 14 over thecore, such as during spinal extension and/or in the event of maximumlateral flexure.

FIG. 3 illustrates how the disc 10 can also allow for translationalmovement of the plates relative to the core. In the illustratedsituation there has been lateral translation of the plates relative tothe core. The limit of lateral translation is reached when the innerextremity of the flange 28 abuts the base of the groove 54 as indicatedby the numeral 70.

The flange 28 and the groove 54 defined between the ribs 56, preventseparation of the core from the plates. In other words, the cooperationof the retaining formations ensures that the core is held captivebetween the plates at all times during flexure of the disc 10.

In an alternative embodiment, the continuous annular flange 28 may bereplaced by a retaining formation comprising a number of flange segmentswhich are spaced apart circumferentially. Such an embodiment couldinclude a single, continuous groove 54 as in the illustrated embodiment.Alternatively, a corresponding number of groove-like recesses spacedapart around the periphery of the core could be used, with each flangesegment opposing one of the recesses. In another embodiment, thecontinuous flange or the plurality of flange segments could be replacedby inwardly directed pegs or pins carried by the upper plate 12. Thisembodiment could include a single, continuous groove 54 or a series ofcircumferentially spaced recesses with each pin or peg opposing arecess.

In yet another embodiment, the retaining formation(s) could be carriedby the lower plate 14 instead of the upper plate, i.e. the plates arereversed. In some embodiments, the upper (or lower) plate is formed withan inwardly facing groove, or circumferentially spaced groove segments,at the edge of its inner, curved surface, and the outer periphery of thecore is formed with an outwardly facing flange or with circumferentiallyspaced flange segments.

Although the foregoing is a complete and accurate description of theinvention, any of a number of modifications, additions or the like maybe made to the various embodiments without departing from the scope ofthe invention. Therefore, nothing described above should be interpretedas limiting the scope of the invention at it is described in the claims.

1. A prosthetic disc for insertion between adjacent vertebrae, theprosthetic disc comprising: a core having upper and lower curvedsurfaces, wherein each of the curved surfaces comprises a metal; upperand lower plates, each plate having an outer surface which engages avertebra and a metallic inner curved surface which is shaped to slideand translate over one of the curved metallic surfaces of the core,wherein a center of rotation of the core is free to move relative to theupper and lower plates; a peripheral restraining structure on at leastone of the upper plate and the lower plate to hold the core against acurved surface of at least one of the plates during sliding movement ofthe plates over the core, the peripheral restraining structurecomprising an annular structure extending over at least a portion of theperiphery of the core and a ring structure on one of the upper and lowerplates, which ring structure engages and retains the annular structure,wherein the restraining structure is adapted to contact an opposingplate to limit relative inclination of the plates during slidingmovement of the plates over the core.
 2. A prosthetic disc as in claim1, wherein the plates comprise at least one metal selected from thegroup consisting of cobalt chrome molybdenum, titanium and stainlesssteel.
 3. A prosthetic disc as in claim 2, wherein each plate comprisesan MRI compatible material coupled with a hardened material forming theinner surface.
 4. A prosthetic disc as in claim 3, wherein theMRI-compatible material comprises titanium and the hardened materialcomprises cobalt chrome molybdenum.
 5. A prosthetic disc as in claim 3,wherein the MRJ-compatible material comprises titanium and the hardenedmaterial comprises titanium nitride.
 6. A prosthetic disc as in claim 3,wherein the MM-compatible material and the hardened material are coupledtogether using a process selected from the group consisting of welding,lamination, slip fitting, interference fitting, adhesion and heating andcooling one material to attach it to another.
 7. A prosthetic disc as inclaim 1, wherein at least one curved surface of the core comprises atleast one metal selected from the group consisting of cobalt chromemolybdenum, titanium and stainless steel.
 8. A prosthetic disc as inclaim 7, wherein the entire core is metallic.
 9. A prosthetic disc as inclaim 8, wherein the core and both plates comprise the same metal.
 10. Aprosthetic disc as in claim 7, wherein a portion of the core between thecurved surfaces comprises ceramic.
 11. A prosthetic disc as in claim 7,wherein a portion of the core between the curved surfaces comprises apolymer.
 12. A prosthetic disc as in claim 1, wherein the core comprisestwo oppositely facing convex low-friction surfaces which slidably engagethe inner curved surfaces of the upper and lower plates.
 13. Aprosthetic disc as in claim 1, wherein at least one of the curvedsurfaces is spherical.
 14. A prosthetic disc as in claim 13, whereinboth curved surfaces are spherical.
 15. A prosthetic disc as in claim 1,further including at least one surface channel on at least one of theupper and lower curved surfaces of the core for allowing passage ofbodily fluid to promote lubrication between the core and at least one ofthe plates.
 16. A prosthetic disc as in claim 15, wherein the at leastone surface channel comprises a helical channel.
 17. A prosthetic discas in claim 15, wherein the at least one surface channel comprises twoperpendicular channels.
 18. A prosthetic disc as in claim 15, whereineach of the upper and lower surfaces of the core includes at least onesurface channel.
 19. A prosthetic disc as in claim 1 or 15, furtherincluding at least one surface channel on at least one of the innercurved surfaces of the plates.
 20. A prosthetic disc as in claim 19,wherein each of the inner curved surfaces of the plates includes atleast one surface channel.
 21. A prosthetic disc as in claim 1, whereinat least one of the upper and lower curved surfaces of the core includesthreads for movably coupling the core with at least one of the upper andlower plates via complementary threads thereon.
 22. A prosthetic disc asin claim 1, wherein movement of the core within the restrainingstructure is unconstrained.
 23. A prosthetic disc as in claim 1, whereinthe peripheral restraining structure defines a stop structure to limitrelative inclination of the plates during sliding movement of the platesover the core.
 24. A prosthetic disc as in claim 1, wherein theperipheral restraining structure defines a stop structure to limit axialrotation of the plates during sliding movement of the plates over thecore.
 25. A prosthetic disc as in claim 1, wherein the peripheralrestraining structure will engage one side of the core to lift anopposite side of the core during sliding movement of the plates over thecore.
 26. A prosthetic disc as in claim 1, wherein the ring structurecomprises a flange which defines an overhang, and wherein at least partof the annular structure on the core extends into the overhang toprovide an interference fit of the core with the flange.
 27. Aprosthetic disc as in claim 26, wherein the annular structure comprisesa rim which extends continuously around a lateral circumference of thecore, the rim having a greater width at least some locations than awidth of an inner edge of the flange to provide said interference fit.28. A prosthetic disc as in claim 1, wherein the outer surfaces of theupper and lower plates have at least one surface feature for promotingattachment of the outer surfaces to the vertebrae.
 29. A prosthetic discas in claim 28, wherein the at least one surface feature comprises aplurality of serrations disposed along the outer surfaces.
 30. Aprosthetic disc as in claim 29, wherein the at least one surface featurefurther comprises a surface coating.
 31. A prosthetic disc as in claim30, wherein the surface coating comprises plasma sprayed titanium.
 32. Aprosthetic disc as in claim 29, wherein the at least one surface featurefurther comprises a plurality of concavities formed by aluminum oxideblasting.
 33. A prosthetic disc as in claim 29, wherein the at least onesurface feature further comprises at least one fin disposed on each ofthe outer surfaces.
 34. A prosthetic disc as in claim 1, wherein thecore has a height between the upper and lower curved surfaces and awidth, and wherein a core maximum height is smaller than the core width.35. A prosthetic disc for insertion between adjacent vertebrae, theprosthetic disc comprising: a core having upper and lower curvedsurfaces, wherein each of the curved surfaces comprises a metal; upperand lower plates, each plate having an outer surface which engages avertebra and a metallic inner curved surface which is shaped to slideand translate over one of the curved metallic surfaces of the core,wherein a center of rotation of the core is free to move relative to theupper and lower plates; and a peripheral restraining structure on atleast one of the upper plate and the lower plate to hold the coreagainst a curved surface of at least one of the plates during slidingmovement of the plates over the core, wherein the restraining structureis adapted to contact an opposing plate to limit relative inclination ofthe plates during sliding movement of the plates over the core; whereinthe peripheral restraining structure comprises: an annular structureextending over at least a portion of the periphery of the core, and aring structure on one of the upper and lower plates, wherein the annularstructure has a diameter sized to pass through the ring structure, andwherein the core is retained within the ring structure due to forcesplaced on the prosthetic disc via the adjacent vertebrae.
 36. Aprosthetic disc for insertion between adjacent vertebrae, the prostheticdisc comprising: a core having upper and lower curved surfaces, whereineach of the curved surfaces comprises a metal; upper and lower plates,each plate having an outer surface which engages a vertebra and ametallic inner curved surface which is shaped to slide and translateover one of the curved metallic surfaces of the core, wherein a centerof rotation of the core is free to move relative to the upper and lowerplates; and a peripheral restraining structure on at least one of theupper plate and the lower plate to hold the core against a curvedsurface of at least one of the plates during sliding movement of theplates over the core, wherein the restraining structure is adapted tocontact an opposing plate to limit relative inclination of the platesduring sliding movement of the plates over the core; wherein theperipheral restraining structure comprises: a groove around a lateraledge of the core; and a ring structure on at least one of the upper andlower plates, said ring structure having a lip which engages the groove.37. A prosthetic disc as in claim 36, wherein the lip extends into thegroove to form an interference fit between the ring structure and thecore.
 38. A prosthetic disc for insertion between adjacent vertebrae,the prosthetic disc comprising: a metallic core having upper and lowercurved metallic spherical surfaces; first and second metallic plates,each plate having an outer surface which engages a vertebra and an innercurved spherical surface which slides and translates over the curvedspherical surface of the core, wherein a center of rotation of the coreis free to move relative to the upper and lower metallic plates; and aperipheral restraining structure on at least one of the first and secondplates to hold the core against a curved surface of at least one of theplates during sliding movement of the plates over the core, theperipheral restraining structure comprising a peripheral structureextending over at least a portion of the periphery of the core and aprojecting structure on one of the first and second plates, whichprojecting structure engages and retains the peripheral structure,wherein the restraining structure is adapted to contact an opposingplate to limit relative inclination of the plates during slidingmovement of the plates over the core.
 39. A prosthetic disc as in claim38, further including at least one surface channel on each of the upperand lower curved surfaces of the core for promoting flow of lubricatingfluid between the core and the plates.
 40. A prosthetic disc as in claim39, further including at least one surface channel on each of the innercurved surfaces of the first and second plates for promoting flow oflubricating fluid between the core and the plates.
 41. A prosthetic discas in claim 38, wherein the core and the plates comprise at least onemetal selected from the group consisting of cobalt chrome molybdenum,titanium and stainless steel.
 42. A prosthetic disc as in claim 41,wherein the core and the plates comprise the same metal.
 43. Aprosthetic disc as in claim 38, wherein the core has a height betweenthe upper and lower curved surfaces and a width, and wherein a coremaximum height is smaller than the core width.
 44. A prosthetic disc asin claim 38, wherein the projecting structure comprises an inwardlyprojecting ring.
 45. A method for implanting an intervertebral discprosthesis between adjacent vertebrae, the method comprising: implantingan upper metallic plate against a lower surface of an upper vertebralbody; implanting a lower metallic plate against an upper surface of alower vertebral body; disposing a core between the upper and lowerplates, the core having curved surface comprising a metal, whereindisposing the core between the plates comprises passing an annularstructure on the core through a ring structure on at least one of theplates, and wherein the annular structure has a diameter sized to passthrough the ring structure to restrain peripheral movement of the core;and allowing the core to float with a mobile center of rotation betweenspherical cavities in each of the upper and lower plates.
 46. A methodas in claim 45, wherein implanting each of the plates comprises slidinga fin on each plate into a corresponding groove formed in its respectivevertebral body.
 47. A method as in claim 46, wherein sliding the fin isperformed in a direction from posterior to anterior.
 48. A method as inclaim 46, wherein sliding the fin is performed in a lateral direction.49. A method as in claim 46, wherein sliding the fin is performed inangled direction between posterior-anterior and lateral.
 50. A method asin claim 46, wherein implanting further comprises contacting texturedouter surfaces of the upper and lower plates with the upper and lowersurfaces of the vertebral bodies.
 51. A method as in claim 45, whereinthe core has a height between the upper and lower curved surfaces and awidth, and wherein a core maximum height is smaller than the core width.52. A prosthetic disc for insertion between adjacent vertebrae, theprosthetic disc comprising: a core having upper and lower bearingsurfaces, wherein each of the bearing surfaces comprises a metal; upperand lower plates, each plate having an outer surface which engages avertebra and a metallic inner bearing surface which is shaped to slideand translate over one of the bearing metallic surfaces of the core,wherein a center of rotation of the core is free to move relative to theupper and lower plates; a peripheral restraining structure on at leastone of the upper plate and the lower plate to hold the core against abearing surface of at least one of the plates during sliding movement ofthe plates over the core, the peripheral restraining structurecomprising an annular structure extending over at least a portion of theperiphery of the core and a ring structure on one of the upper and lowerplates, which ring structure engages and retains the annular structure,wherein the restraining structure is adapted to contact an opposingplate to limit relative inclination of the plates during slidingmovement of the plates over the core.
 53. A prosthetic disc as in claim52, wherein at least one bearing surface of the core comprises at leastone metal selected from the group consisting of cobalt chromemolybdenum, titanium and stainless steel.
 54. A prosthetic disc as inclaim 53, wherein the entire core is metallic.
 55. A prosthetic disc asin claim 54, wherein the core and both plates comprise the same metal.56. A prosthetic disc as in claim 52, wherein the core comprises twooppositely facing convex low-friction surfaces which slidably engage theinner bearing surfaces of the upper and lower plates.
 57. A prostheticdisc as in claim 52, wherein at least one of the bearing surfaces of thecore is spherical.
 58. A prosthetic disc as in claim 52, wherein bothbearing surfaces of the core are spherical.
 59. A prosthetic disc as inclaim 52, wherein at least one of the upper and lower bearing surfacesof the core includes threads for movably coupling the core with at leastone of the upper and lower plates via complementary threads thereon. 60.A prosthetic disc as in claim 52, wherein movement of the core withinthe restraining structure is unconstrained.
 61. A prosthetic disc as inclaim 52, wherein the peripheral restraining structure defines a stopstructure to limit axial rotation of the plates during sliding movementof the plates over the core.
 62. A prosthetic disc as in claim 52,wherein the peripheral restraining structure will engage one side of thecore to lift an opposite side of the core during sliding movement of theplates over the core.
 63. A prosthetic disc as in claim 52, wherein thering structure comprises a flange which defines an overhang, and whereinat least part of the annular structure on the core extends into theoverhang to provide an interference fit of the core with the flange. 64.A prosthetic disc as in claim 63, wherein the annular structurecomprises a rim which extends continuously around a lateralcircumference of the core, the rim having a greater width at least somelocations than a width of an inner edge of the flange to provide saidinterference fit.